Adhesive compositions such as those described in U.S. Pat. No. 3,335,041 to Osborne, which contain an organic solvent, natural rubber and polymers of conjugated dienes, have been used to bond tire tread stock. The use of organic solvents in these compositions has certain disadvantages. For example, the organic solvents pose a serious fire hazard. Also, the solvents are a serious health hazard to those that come in contact with the fumes. Moreover, when solvent recovery is required, this is an expensive operation, particularly with respect to equipment investment.
In view of the hazards associated with the use of organic solvents, it would be desirable to eliminate or at least substantially reduce the amount of organic solvent present in adhesive compositions which are used for bonding, for example, tread stock to a carcass portion during the building of a tire.
Aqueous compositions have been used heretofore for bonding polyester tire cords to rubber compounds. For example, U.S. Pat. No. 3,843,484 to Kamiyoshi et al describes a method for bonding a synthetic fibrous material to rubber by applying an aqueous dispersion containing a novolak resin which is derived from the reaction of a monohydroxy benzene and formaldehyde, a precondensate derived from the reaction of resorcinol and formaldehyde and a rubber latex. U.S. Pat. No. 3,857,730 to Kalafus et al describes a process for bonding rubber compounds to polyester reinforcing elements by employing a one-step dip process in which the dip consists essentially of an alkaline aqueous emulsion of a rubbery vinyl pyridine copolymer containing butadiene-styrene and 2-vinyl pyridine and a heat reactable 2,6-bis(2,4-dihydroxy phenylmethyl)4-chlorophenol composition. U.S. Pat. No. 4,026,744 to Elmer also discloses a method for bonding rubber compounds to reinforcing elements by using a dip process in which the dip contains an alkaline aqueous dispersion of a mixture of a rubbery vinyl pyridine/styrene/butadiene terpolymer and a lignin sulfonate-resorcinol-formaldehyde reaction product. U.S. Pat. No. 4,285,756 to Elmer describes an aqueous alkaline dispersion containing a rubbery polybutadiene and a water-soluble heat reactive phenolic resin which is useful as an adhesive for bonding polyamide or polyester reinforcing elements to ethylene-propylene-diene rubbery polymer compounds.
Although the above references disclose latex emulsions for bonding reinforcing elements such as tire cords to rubber compounds, these references do not describe a process for preparing aqueous emulsions containing natural and/or synthetic rubber compositions which may be employed as adhesive compositions for bonding tread stock to tire carcasses.
Therefore, it is an object of the present invention to provide an aqueous adhesive composition for bonding unvulcanized rubber compounds. Another object of this invention is to provide an aqueous emulsion for bonding unvulcanized rubber compounds. Still another object of the present invention is to provide a method for bonding unvulcanized rubber compounds together. A further object of this invention is to provide a method for bonding rubber compounds together using an aqueous adhesive composition.